1. Field of the Invention
This invention relates to beam directors for optical fiber coupling and, more particularly, to such apparatus for sequentially directing a laser beam to a plurality of fibers or optical fiber bundles.
2. Description of the Related Art.
Laser distributors have been built using wedge prisms, rhomboid prisms, acousto-optic cells and optical fibers, to name a few examples, as the distributor elements.
A wedge prism is used to change the propagation direction of a beam of light by a fixed angle, and the plane of that angle can be rotated by rotating the prism about the axis of the beam. Thus, it can direct the output beam along any path at a fixed angle from its original propagation direction. A second wedge prism can be used in conjunction with the first to enable beam steering through a continuous two-dimensional range of angles, so that a two-dimensional array of outputs can be accessed from a single beam directed to the apparatus. In such an arrangement, the wedge prisms alter the input beam by changing both the location and direction of the output. The set of possible output beams that a pair of wedge prisms can produce from a given input beam each have a unique direction and location and do not even pass through a common center or junction point. There is, therefore, no simple way to prevent even minor in the prism positioning from causing the focus of the beam to stay from the center of the output fiber core.
A rhomboid prism can be used to produce an output beam that is exactly parallel to the input beam, but having a laterally displaced axis. By using a proper mounting arrangement and rotating the rhomboid prism about the laser beam axis, the beam can be directed into any one of a number of output devices. The rhomboid prism possesses the property that it can produce an output beam that is parallel to the input beam, but having an axis displaced laterally, relative to the input beam. Thus it can be used as the moving distributor element in a system that is highly error tolerant, provided that the input beam is parallel to the optical axes of the output fibers and the lenses used to focus the distributed beam into the fibers, and also provided that the beam displacement results in the prism output beam falling entirely inside the open aperture of the respective lenses. Such apparatus, however, presents the disadvantage that the prism must be mounted between the laser and the lenses, making it difficult to maintain the required parallelism between the laser beam and the lens axes.
A typical acousto-optic cell consists of a transparent block of material that is attached to a piezoelectric transducer which is in turn driven by a high frequency electric oscillator. The acoustic waves introduced by the piezoelectric transducer into one side of the transparent block cause the formation of alternating layers of compression and dilatation in the block. The set of layers acts as a diffraction grating where the spacing between the layers can be used to control the angle at which the laser beam emerges from the block.
Acousto-optic cells offer very rapid sequencing from one output to another, but require very high frequency oscillators that can be rapidly and cleanly switched from one frequency to another. The system must be temperature compensated at each of the separate oscillator frequencies, must not be affected by radio frequency radiation in the environment, and cannot easily be made error tolerant In such a system, the optical components that are distant from each other must be precisely mounted and rigidly held to function properly. The acousto-optic laser distributor is thus comparatively complex and expensive, and it may be unreliable in severe environments.
A retroreflector prism, sometimes referred to as a trihedral retroreflector or corner cube reflector, has the property that any ray entering the effective aperture will be reflected internally and will emerge from the entrance/exit face parallel to itself but with an opposite direction of propagation. This property of retroreflection with parallelism is, within acceptance angle limits, independent of the orientation of the retroreflector. A retroreflector prism therefore may be appropriate for use in situations where orientation is difficult or impossible to control and where a mirror would therefore be unsatisfactory.
Retroreflector prisms present certain theoretical limitations which may adversely affect their desirable properties of total internal reflection (TIR) and reflected beam parallelism. However, retroreflector prisms are commercially available which have the capability of compensating for such inherently adverse properties and can assure a parallelism between incident and returned beams of two arc seconds or better. Such retroreflector prisms are entirely suitable for inclusion in embodiments of the present invention.